Low dielectric composite substrate

ABSTRACT

The cracking experienced during thermal cycling of metal:dielectric semiconductor packages results from a mismatch in thermal co-efficients of expansion. The non-hermeticity associated with such cracking can be addressed by backfilling the permeable cracks with a flexible material. Uniform gaps between the metal and dielectric materials can similarly be filled with flexible materials to provide stress relief, bulk compressibility and strength to the package. Furthermore, a permeable, skeletal dielectric can be fabricated as a fired, multilayer structure having sintered metallurgy and subsequently infused with a flexible, temperature-stable material to provide hermeticity and strength.

CONTINUING APPLICATION DATA

This application is a continuation in part application of U.S. patentapplication Ser. No. 07/167,606, filed Mar. 11, 1988, entitled "LowDielectric Composite Substrate", now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of microelectronics and inparticular to semiconductor device packaging.

BACKGROUND OF THE INVENTION

In the field of microelectronic fabrication for computer applications,there is an ever-increasing demand for faster components. Thesemiconductor devices, themselves, are being continuously upgraded toincrease speed, however, it is estimated that one-half of the processingtime is taken up in inter-connection and power distribution circuitry.The delays encountered in the electronic package are therefore ascritical to the overall performance time as are the device speeds.Furthermore, the reliability and useful life of components are concernswhich must be addressed. One must additionally keep in mind the physicalrequirements and restrictions imposed by the different materials used inthe overall component package. The properties desired in a semiconductorpackage include a thermal coefficient of expansion which is compatiblewith that of the devices connected thereto and the combination of a lowdielectric, highly insulative material with internal conductors of highconductivity. Further physical properties which are desirable includehigh strength and toughness and a minimum of distortion of featuresduring processing. The current packaging technology relies upon ceramicsas the insulative materials with compatible metallurgy, such as aluminasubstrates with molybdenum lines and vias. The metallurgicalrequirements include fair conductivity and thermal stability at theprocessing temperatures necessary to fabricate the ceramic package.

The combination of highly insulative material, having a relatively lowthermal expansion coefficient, with a good conductor, which will have agreater thermal expansion, results in tremendous stresses created in thematerials during high temperature processing. Cracking can consequentlyoccur in the insulative material. If further processing steps areperformed, solvents can permeate the cracks in the package giving riseto reliability concerns.

One proposed solution to the cracking problem is to overcoat thesubstrate with an inorganic sealing layer, as taught in IBM TechnicalDisclosure Bulletin, Vol. 15, No. 6, page 1974 (Nov. 1972). Anotherarticle teaches filling the cracks by overcoating the ceramic with aninorganic dielectric layer and subsequently machining away the excessdielectric, down to the ceramic surface (See, IBM Technical DisclosureBulletin Vol. 16, No. 2, page 624 (Jul. 1973). However, the proposedsolutions to the crack problem do not address the fact that thesubstrate to metallurgy mismatch still exists and that further crackingmay well occur due to temperature excursions encountered duringprocessing steps conducted subsequently, such as device joining. When anoverlayer sealing approach is used, thermal expansion mismatch may alsobe encountered between the substrate and the dielectric overlayerrendering the sealing layer itself susceptible to cracking andconsequent permeability. Additionally, delamination concerns arisebetween a fully sintered body and a subsequently deposited glassovercoat. When using a full overlayer for sealing, the overcoat must beetched and screened to form metal vias connecting to the underlyingmetallurgy. The etching process may expose permeable areas of thesubstrate to adverse solvents and conditions. Subsequent metaldeposition into those via holes, also, does not guarantee connectivityand, therefore, conductivity. Consequently, the delamination concern iscompounded by the potential for creating an open in the metal contactshould the metallized sealing layer expand and/or delaminate.

Attempts have been made to address the mismatch problem by matching thethermal coefficients of expansion (hereinafter TCE's) of the associatedmaterials. However, sacrifices must then be made with regard to otherequally desirable characteristics, such as the dielectric of theinsulative material and the conductivity of the metallurgy.

Promoting the adhesion of the abutting materials is still anotherapproach to the mismatch problem. One approach, taught in Japanesepatent application 60-096586 of Hitachi Metal KK, is to enhance theadhesion of the abutting materials by increasing the contact surfacearea. Adhering the materials will not prevent cracking, however. Rather,the good adhesion of these materials can transfer the stresses causingcracks to propagate beyond the boundaries of the adhered materials andinto the body of the ceramic located between the metal features.

Still another approach is to enhance the mechanical integrity of theceramic, using known techniques to increase the crack resistance, i.e.toughness, of the ceramic, as taught in patent application Ser. No.892,687, filed Aug. 1, 1986, and assigned to the present assignee. Thetoughening approach is effective; however, the dielectric properties ofthe ceramic may be adversely affected by the inclusion of tougheningagents.

On the other hand, in seeking the ideal dielectric properties,sacrifices are made with regard to both the mechanical integrity of thesubstrate and its thermal properties. The art is replete with techniquesfor adjusting the dielectric constant (hereinafter, K) of a devicesubstrate to decrease the capacitance and thereby increase the speed oftransmission through the associated metallurgy. IBM Technical DisclosureBulletin, Vol. 20, No. 12, page 5174 (May 1978) teaches the placement ofspacers to provide for a layer of air as dielectric (having a K of 1) inseries with the glass dielectric for a multilayer module. Anotherapproach is to intersperse air throughout the glass or ceramicdielectric itself. A method for accomplishing this is taught in IBMTechnical Disclosure Bulletin, Vol. 14, No. 9, page 2581 (Feb. 1972)wherein a foam-like glass having controlled amounts of microscopic voidsis provided as the low dielectric substrate material. Still anotherpublication, Japanese Patent application 59-111345 teaches thedispersion of hollow spherical powders into the raw ceramic slurry. Saidhollow spheres remain intact after low temperature sintering to providea K of 1 in "solution" with the ceramic dielectric value. Freeze-driedformation of hollow (or air-filled) alumina macropores is the subject ofJapanese Patent 59-196740 to Kiyatama Koygakk. Each of the foregoingteachings discloses air-filled, non-permeable ceramic voids. Althoughadequate as low dielectric materials, the resulting substrates will beincapable of withstanding the thermal and tensile stresses of devicejoining. Furthermore, the spheres created will still be susceptible tocracking as a result of the thermal expansion mismatch. Moreover, evenin the absence of cracking, the interstices of the voids/spheres may bepermeable to processing solvents.

It is therefore an objective of the present invention to provide animpermeable substrate, having a low dielectric constant, for devicemounting.

It is a further objective of the subject invention to provide asubstrate, of low dielectric insulative material and internal metallurgyof high conductivity, which will be impervious to the effects of thermalexpansion mismatch.

It is still another objective of the subject invention to provide amaterial which can be incorporated into an electronic packagingsubstrate to provide a flexible, hermetic link between the associatedmaterials.

It is yet another objective of the present invention to teach a methodfor obtaining a uniform, fillable void in a substrate structure whereinthe dielectric and metal materials are bonded by a flexible, hermeticlinking agent introduced into that void.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objectives are realized by the subject invention whereinthe first embodiment teaches the filling of unwanted gaps and cracks ina substrate by means of impregnation of gaps, etc., with a polymericfill material. The second embodiment teaches the fabrication of anentirely new composite multilayer substrate wherein a skeletal ceramicnetwork having fixed metallurgical features is completely filled with apolymeric material. The second embodiment yields a ceramic and polymericsubstrate having a low dielectric constant with fixed metal features ofhigh conductivity, wherein the polymeric material provides the necessarybulk compressibility to yield to the associated materials as theyundergo thermal expansion, thereby eliminating the stresses on theceramic.

The invention will be further described with reference to the includeddrawings wherein:

FIG. 1 illustrates a sintered ceramic substrate having metal featurestherethrough, and illustrates cracking of the ceramic due to the thermalstresses encountered during sintering and other thermal cycling.

FIG. 2 illustrates a ceramic-metal substrate wherein the bonding at theinterfaces of the abutting materials has been enhanced and thereforecracks have been generated in areas removed from the interfaces.

FIG. 3 illustrates one embodiment of the subject invention whereby thecracks in the ceramic are filled.

FIG. 4 is an enlarged view of the structure of FIG. 3 after one fillingstep in accordance with the subject invention.

FIG. 5 illustrates an enlarged view of the structure of FIG. 3 afterseveral filling steps.

FIG. 6A, 6B and 6C illustrate a ceramic and metal structure wherein thematerials are not abutting; the structure is fabricated with fugitivepaste (6A) which forms a uniform gap (6B) to be filled (6C) inaccordance with one of the inventive teachings.

FIGS. 7A, 7B, and 7C illustrate individual greensheets havingmetallurgical features which are at least partially encapsulated byporous dielectric material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As discussed above, the most desirable qualities for a semiconductorpackage include a low dielectric constant, high mechanical integrity andgood thermal stability. The required values for dielectric constant andTCE are determined, in part, by the values and/or properties of thematerial, generally silicon or gallium arsenide, used in fabricating thesemiconductor devices to be mounted thereon. Matching the thermalexpansion coefficient of the substrate and devices eliminatesthermally-induced fatigue failures of the interconnections, therebyincreasing the life and the reliability of the component. It is alsoadvantageous to match the dielectric constant of the associatedmaterials to eliminate any deleterious capacitance effects. The lowpermittivity of the insulative packaging material enhances speed throughthe internal metallurgy, as is quantitatively defined by the relation ofthe delay being proportional to the square root of the dielectricconstant of the package. In the industry, an alumina substrate, having athermal coefficient of expansion similar to that of silicon, withcompatible metallurgy of molybdenum, has been used. Regardless of thespecific materials used, however, there will be a thermal expansionmismatch between an insulative material, illustrated as 11 in FIG. 1,and the metallurgy, illustrated in FIG. 1 as 12. If theinsulator/metallurgy package is subjected to thermal cycling duringprocessing, the mismatch in thermal expansion will manifest itself bygenerating cracks, 13, in the ceramic. The existence of cracks which areopen to the top surface of the ceramic renders the substrate permeableto processing solutions and ambients, the presence of which couldcompromise the integrity of, if not destroy, the package. As notedabove, prior art solutions to the permeability problem have notadequately addressed the root problem or have introduced new variableswhich otherwise degrade the overall characteristics of the product. FIG.2 illustrates the approach of enhancing the adhesion of the ceramic tothe metallurgy. As is evident therein, at 23, cracking of the ceramiccan still occur. The enhanced ceramic-metal bond, 24, may not break;however, this point will be the locus of highest tension during thermalcycling. Since the ceramic-metal bond will not itself crack, the thermalexpansion mismatch will place the surrounding ceramic under tremendousstress. That stress can cause the ceramic to crack at points away fromthe bond and those cracks will be generated out to the surface of thesubstrate as illustrated at points 23. Therefore, it is imperative toaddress the problem by matching TCE's or otherwise compensating for theTCE mismatch and the stresses resulting therefrom. The present inventionincludes a remedy for the problems encountered. In addition, the presentapplication teaches and claims a new technology including a lowdielectric composite structure, and methods for producing the same,wherein the TCE and K incompatibility problems encountered by thepackaging industry are avoided.

Specifically, prior art substrates can be "salvaged" by the use of theremedial embodiments claimed herein. Furthermore, a new compositestructure is taught which can replace crack-prone ceramic/metalpackages.

What is herein addressed is the need to provide a remedy in the form ofa flexible link between the TCE-incompatible packaging materials foundin the semiconductor packaging industry today. As discussed in theBackground section, there are prior art teachings directed to fillingcracks created by the stresses experienced during thermal cycling. Theremedial materials suggested, however, are glass or other inorganicdielectrics, which presumably have similar thermal properties to thoseof the insulative material. Again, the possibility of re-cracking or newcracking is not addressed. What is needed is a structure wherein aflexible fill material is chosen to provide the bulk compressibility, oranalogous mechanical properties, to absorb the stresses, or to translatethe stresses away from the ceramic. What is herein proposed is the useof a temperature-stable polymeric material to fill the gaps and/orcracks which are created during high temperature processing of theceramic package. Specifically, the polymer fill material must be stableto the temperatures encountered in the processing of the packagesubsequent to infusion of the fill material.

In accordance with known package processing, such as is taught in U.S.Pat. Nos. 3,770,529, 4,234,367 and 4,301,324, incorporated by referenceherein, ceramic, glass-ceramic or alumina greensheets are cast,patterned, and then metallized. The metallized greensheets are thenstacked and sintered at extremely high temperatures. As discussed in theaforementioned TDB articles, it is during the heating and coolingassociated with the sintering step that cracks may occur in the ceramic,as illustrated in FIG. 1, at 13. Post-sintering process steps, such asdevice joining, may involve the additional use of high temperatureswhich may promote more cracking or the propagation of existing cracks,and may further involve the use of processing solvents which canpermeate the cracks in the substrate and interfere with subsequentprocessing. Trapped solvents may also interfere with the functioning ofthe finished package. Therefore, it is most advantageous to fill thecracks immediately after the sintering has been completed. FIG. 3illustrates the substrate of FIG. 1 after a crack-filling step has beenperformed. The cracks therein, 33, have been filled with a polymericmaterial, 35. The polymer is introduced into the substrate by use ofscreening, vacuum, or analogous impregnation processes in order toassure that all points of exposure have been filled. The exact processby which the polymer is introduced into the substrate is not a criticalpart of the present invention, as one having ordinary skill in the artcan ascertain methods to obtain the desired result in addition to thesuggested methods of melt flow, soaking by capillary action, vacuumimpregnation by evacuating and soaking, filling by vapor transport andfilling by injection molding. The desired properties which the fillmaterial must possess include excellent adhesion to the metal and thesubstrate material, high thermal and oxidative stability, and goodresistance to humidity. Suitable materials including thermally-stablesilicon-containing polymers such as silanes, siloxanes, silazanes,organosiloxanes, and polyimides, epoxies and bismaleimides. As onehaving skill in the polymer art may ascertain, the polymeric materialmust be sufficiently fluid at a given temperature, for vacuumimpregnation, soaking, melt flow or injection molding, to allow forimpregnation of the voids and must then be curable upon treating,preferably heat curing, in situ. As an example, and with reference toFIG. 4, one may introduce a temperature-stable polymer in solution, suchas a BTDA-APB derived polyimide in appropriate solvent to fill thecracks, 33. The structure may then be heated, to between 125°-165° C.,to drive off the solvents and to cure or imidize the polymer, in situ inaccordance with known polymer processing requirements. When a solventsystem is used, volume loss may be experienced during the step ofdriving off the solvent. If the remaining volume of cured polymer hassufficiently "plugged" the exposed surfaces, 37, of the substrate, andis thereby providing the desired hermeticity, no further processing isnecessary. The vacuum-filled voids within the three dimensional matrixof the cured polymer, 35, will serve to absorb the tensile stressesencountered during thermal cycling. If, however, the surface of thesubstrate is not fully sealed upon the completion of the initial curing,the vacuum impregnation, solvent removal and polymer curing steps may berepeated until a more complete fill is achieved, see FIG. 5. In theinstance where the voids in the ceramic have been completely filled,i.e. fully dense with the polymer, the polymer will still operate toreduce the stress on the ceramic, 31. Specifically, rather than theabove-discussed mechanism whereby the vacuum-filled voids in the polymerprovide bulk compressibility, here the dense polymer itself, 35, willabsorb the stresses and, due to its superior mechanical/elasticproperties, translate the x-y components of stress to an expansion ofthe polymer alone. For example, where the polymer is continuous in thez-direction to the surface of the substrate, as in FIG. 5 at 37, the x-ycomponents will be translated into z-direction extrusion of the polymerat that unobstructed surface, effectively reducing stress on theceramic. A sample fill system which has been applied in thick films to acordierite and copper system was a silicon-containing system comprising1,1 polydimethylsilazane in n-butyl acetate (1:1 ratio). After a 500°bake in N₂, the amorphous, glass-like film exhibited superior adhesionto the substrate, with no cracking evident upon thermal cycling.

The voids in the ceramic can be filled not only by the repeated fill andcure steps using a solvent-based system, but also by direct introductionof the polymer to the voids. This may be accomplished either by using asystem wherein the solvent reacts with the solute upon curing therebyyielding little or no volume loss, such as an epoxy system, or byproviding a polymer which can be introduced as a pure vapor or fluid andsubsequently cured in situ. Again, the fill material should possess thedesired properties, including high thermal and oxidative stability andgood adhesion to the ceramic and the metal. A further desirable propertyof the fill material is to effect curing without the need for exposureto the atmosphere for outgassing, since the bulk of the polymer will notbe exposed to the surface atmosphere. In order for the fill material topenetrate the most minute fissures, it is desirable to use a materialwith a relatively low viscosity and low surface tension. Not only is itnecessary for the fill material to be thermally stable, but it must alsobe inert to any processing solvents to which it may be exposed in thesubsequent package fabrication steps. It is also desirable, of course,to have a low dielectric constant material with a low TCE. The TCE ofmost of the applicable polymeric materials is higher than that of eitherthe dielectric material or the internal metallurgy which is commonlyused for packaging applications; however, since the polymers have therequisite mechanical properties, the fill material can effectivelydissipate the stress of its own expansion along with the components ofstress imparted by the expanding dielectric and metallurgy, the neteffect of which is reduction of stress in the package. Suitable polymersinclude not only organic polymers such as epoxies, polyimides,bismaleimides and acetylenes but also inorganics such as silanes,silazanes and siloxanes, and organosiloxanes such as silicones. Ingeneral, it has been found that linear polymers do not maintain theirintegrity upon subsequent thermal cycling. A preferredsilicon-containing polymer, then, would be a sesquisiloxane or silazanewherein the polymer, upon curing, would cross-link and not depolymerizeduring the thermal excursions encountered in chip joining and relatedprocesses.

Needless to say, in the usual course of fabrication, the cracksgenerated in the ceramics are non-uniform and non-linear. If a crack hasnot propagated directly out to the surface of the substrate, it maynevertheless be possible to fill the crack by means of the vacuumimpregnation techniques. However, such an encapsulated crack in the bodyof the ceramic, which has been completely filled with a high expansionpolymer cannot relieve the stress on the ceramic. Rather, it willintensify the stress when the polymer attempts to translate thestresses, having no unobstructed, surface facing for expansion. Ideally,one would like to have a substrate, as illustrated in FIG. 6A-C, whereina uniform gap is created between the ceramic and the metallurgy. Thisuniform gap will allow the expansion of the materials without anyattendant stress on the low fracture strength ceramic. By filling theuniform gap with a polymer, either with the techniques described aboveor by more traditional fluid flow techniques, one can join the substratematerials, seal the exposed surfaces and provide the bulk compressiblematerial to absorb, or the elastic material to translate, the thermalexpansion stresses. A substrate fabrication process for achieving such auniform gap involves the use of a double screening technique whenapplying the metallurgy to the punched greensheet. Specifically, afterpunching, a first screening is performed to coat the surfaces of the viahole, 67 in FIG. 6A, with a "fugitive paste", 64. After the coatinglayer is dried, the metallurgy, 62, is screened into the remaining voidspace in the via hole, yielding the structure illustrated in FIG. 6A.The substrate is then sintered in accordance with the known technology.The fugitive paste, 64, is chosen to provide a structural frame duringthe screening process but to burn off during the sintering step,preferably at a temperature higher than that at which the metallurgy hassintered and the ceramic has densified. A sample paste for this purposeis terephthalic acid which will coat the via walls for screeningpurposes and maintain structural integrity through the binder burn outtemperatures taught in Kumar 4,301,324 and Herron 4,234,367, but willburn off during sintering. The resulting structure, as illustrated inFIG. 6B, has fully sintered metallurgy standing freely in alignment withthe surrounding ceramic. The substrate may then be subjected to afilling step by which the suitable polymer is impregnated into theuniform gaps, 65 in FIG. 6C. Another embodiment of this process would beto provide, upon the first screening, a coating material which willremain as a porous collar about the metallurgy after sintering. Thisalternate material must provide the structural and thermal integrity towithstand the sintering step and must be porous upon sintering so thatit may be filled with a polymer to, again, provide the mechanicalproperties desired with reference to TCE mismatch. The coating materialshould also promote adhesion of the associated materials and not degradethe conductivity nor compromise the insulative qualities of theassociated materials. An example of such a coating material is a collarof metal paste, of the same metallurgy to be used in the via itself,along with a sintering retardant; so that, during the sintering step,the paste will not sinter but will remain as a porous, fillable metalcollar about the pure metal via. An example for a high conductivity viastructure is a pure copper core via with a copper plus alumina collarmaterial. The collar material will not sinter but will remain porous and"fillable". The proposed collar material will additionally promoteadhesion between the copper via and the oxide ceramic which willcontribute to less permeability and therefore fewer sites requiringinfusion of the backfill material.

Uniform fillable gaps or unmetallized via holes or grooves could beprovided in the substrate body in addition to or in lieu of thoseproposed to be located at the metal via holes. Any fillable void, whichhas an unobstructed exposed surface and which has been filled with asuitable polymer, should provide relief from the TCE mismatch stresses.It appears to be most advantageous, however, to provide the uniform,fillable gaps in alignment with the metal via holes in order tofacilitate the fabrication steps. In addition, the polymer can promoteadhesion between the associated materials if it is located therebetween.Intervening polymer may serve as an efficient insulating means adjacentto the conductors; polyimide, for example, having a dielectric constantof 2.5-3.0. Furthermore, the abutting relationship of the polymer to theexpanding materials will allow the polymer to more effectively absorband/or translate the attendant stresses. If the polymer is separatedfrom the point of maximum stress, the intervening expanse of ceramic maywell crack before the stress could be alleviated at the polymer-ceramicinterface.

A further alternative to the use of a uniform fillable gap or a uniformfillable porous metal collar is the use of a via material which isentirely porous, and therefore fillable. Since the expansion of a puremetal can cause shearing and resultant cracking, a metallic paste isused to fill the via. Specifically, a composite via may be fabricatedfrom a mixture of a ceramic or a ceramic and glass and the conductivemetallurgy, such as copper. The conductivity of the via may becompromised slightly in favor of the more favorable adhesion andexpansion characteristics. Upon sintering, the composite via willexhibit excellent adhesion to the ceramic via walls. The sinteredcomposite material will have minute but continuous porosity which can bebackfilled in accordance with the foregoing teachings in order to obtainthe desired hermeticity and flexibility. In addition, the compositematerial will have a TCE which is lower than that of pure metal, therebyreducing the chances of stress-induced cracking during subsequentthermal processing. As a fill material, siloxane provides excellentthermal and oxidative characteristics along with favorable viscosity andsurface tension to allow for permeation of minute fissures. It isspeculated that a conductive polymer may be introduced into theinterstices, thereby providing not only a hermetic and flexible link butalso enhanced conductivity in the via.

EXAMPLE 1

A sample substrate may be fabricated in accordance with the followingprocedure shown by way of example. The inventors do not wish to belimited to the exact materials and processes recited herein.

A slurry of crystallizable glass, binders and solvents is mixed and castinto greensheets. The greensheets, when dry, are patterned by punchingand metallized with a pure metal or metal paste. For use with acrystallizable glass ceramic, lower temperature, high conductivitymetals can be used. Examples of appropriate metallurgy include gold,silver, copper, platinum and palladium or alloys thereof. For thecordierite-based system of the example, a copper-based metallurgy isscreened into the pattern of via holes in two screening steps. A first,or collar, paste comprised of copper and alumina is applied to theinside wall of the via hole. Pure copper is then applied to the core ofthe via hole. After lamination of the stacked sheets, the part isinserted into a furnace and subjected to a firing profile as taught inHerron, et al. (U.S. Pat. No. 4,234,367) wherein the copper is sinteredand the glass-ceramic densifies and crystallizes while the copper in thecollar paste is retarded from sintering and remains porous. Aftercooling, the fired substrate is subjected to a vacuum impregnation of aBTDA-APB-derived, high temperature polyimide to fill the porosityassociated with the via collar and any cracking of the ceramic bodywhich may have occurred. The part is then heated to a temperature of125°-165° C. to imidize the polymer.

A preferred embodiment of the invention includes a multilayer substratewhich is not only sporadically backfilled but thoroughly infused withthe polymeric material. The entire substrate body is a composite ofmetallized ceramic and polymer. Such a composite substrate is obtainedby fabricating a skeletal ceramic structure with sintered metallurgicalfeatures and impregnating the entire skeletal structure with a polymer.The skeletal ceramic structure can be fabricated of a ceramic such asalumina, borosilicate or other glass, borosilicate glass plus silica,silica itself, glass-ceramic, or any other suitable substance which canbe fabricated into greensheets, partially or completely sintered into acontinuous yet permeable interconnection network at temperaturessufficient to sinter the associated metallurgy, and backfilled with apolymeric material, and which yields the favorable dielectric propertiesfor semiconductor packaging applications. A sample substrate, withalumina as the dielectric material, can be fabricated initiallyutilizing known techniques; that is, the initial processing steps arethe same as have been previously used (see U.S. Pat. No. 3,770,529). Aslurry of, for example, alumina with solvent and binder can be mixed,cast and dried in accordance with standard processing. The greensheetscan then be punched, blanked and screened with metallurgy as is known.One major point of departure from the present state-of-the-artfabrication techniques can be noted at this stage of processing;specifically, a pure metal can be used without the threat of thecracking or shearing effects encountered during firing, since thealumina will not densify completely. In addition, since the substratewill not be taken to the maximum sintering temperatures, a lower meltingpoint, higher conductivity metal can be used. After stacking andlaminating, the substrate is placed in a furnace having the appropriateambient and the temperature is raised to that temperature at which thechosen metal sinters. If copper is used, there is a preference for anon-oxidizing environment and the sintering temperature will be in therange of 900°-950° C., at which temperatures the inorganic aluminagrains of the substrate body will neck together but will not densify.The resulting substrate will be a partially densified multilayersubstrate having continuous, permeable porosity in the ceramic bodyabout fully sintered metallurgical features. The entire substrate maythen be exposed to an impregnation process by which the flexiblematerial, such as BTDA-APB polyimide or a bismaleimide, is introduced.

In the embodiment wherein the entire substrate is backfilled, the vianeed not be filled with a pure metal, but may be a composite materialwith improved thermal and adhesive properties. If a composite viacomposition is used, it may be desirable to effect two impregnatingsteps whereby the substrate is filled with a first polymeric materialand the interstices in the composite via are subsequently filled with asecond, non-conductive or conductive polymer. The fissures in thecomposite via will be smaller than those found in the substrate.Therefore, a polymer having too great a surface tension to fill theminute fissures of the via interstices may be selected to fill thesubstrate in the first impregnation step. The remaining fissures in thevia could then be filled exclusively with the lower surface tensionconductive polymer.

The proposed process and structure may be implemented for an entireboard or substrate or any increment thereof. In the multilayer ceramiccontext, it is imperative to seal off permeability at the substratesurfaces. It may only be necessary, therefore, to apply the inventiveconcept to the outer surfaces of the substrate. As an example, amultilayer ceramic substrate may be fabricated of a glass-ceramiccomposition in accordance with the teachings found in Kumar, et al U.S.Pat. No. 4,301,324. The greensheets are cast and punched and thenmetallized with a pure copper or a copper-based metallurgy. The outerlayers for the top and bottom surfaces of the substrate will befabricated of the same glass-ceramic composition with a sinteringretardant added. Examples of an appropriate sintering retardant for aglass-ceramic composition of the type taught in Kumar, et al. includebut are not limited to silicon nitride, fused silica, mullite, aluminaand precrystallized cordierite. Additionally, the sintering retardantmay be in the form of a whisker or fiber, for example, silicon nitrideor alumina whiskers. The metallized greensheets of glass-ceramic andsintering retardant are then laminated to the top and bottom surfaces ofthe bulk multilayer ceramic body. The laminate is fired in accordancewith the known teachings (see: Herron, et al U.S. Pat. No. 4,234,367 andKamehara, et al U.S. Pat. No. 4,504,339) to a temperature of 965° C. atwhich the bulk glass-ceramic undergoes densification and crystallizationand the metal or metal-based metallurgy has sintered. The metalassociated with the surface layers co-sinters to the bulk metal but thesurface ceramic does not sinter. It remains porous having formed itscontinuous, permeable network. Infusion of the appropriate fillmaterial, such as a bismaleimide, may then be performed, as above.

Another embodiment of the invention is shown in FIGS. 7A, 7B and 7C.There, one layer of a multilayered ceramic substrate is shown. Normally,of course, there would be a plurality of such layers but for purposes ofillustration, only one such layer is shown. The layer 62 comprisessintered dielectric material. On the layer of sintered dielectricmaterial there is disposed a plurality of patterns of sinteredmetallurgical features. One such metallurgical feature 64 is shown inthe drawings. There is additional porous dielectric material, generallyindicated by 66, selectively disposed over at least one layer of thesintered dielectric material and in contact with the metallurgicalfeatures such that the additional porous dielectric material at leastpartially encapsulates the metallurgical features. That is,metallurgical feature 64 is at least partially encapsulated by theadditional porous dielectric material 66. Since the additional porousdielectric material 66 is selectively disposed over the layer 62 ofsintered dielectric material, the additional porous dielectric material66 does not form a complete layer or coating over layer 62.

The purpose of the additional porous dielectric material is to locallydecrease the dielectric constant of the multilayered ceramic substrate,thereby enhancing the electrical properties of the substrate.

The sintered dielectric material 62 may itself be porous but, in thisembodiment of the invention, it is clearly preferred that it benonporous. In this way, the layers of sintered dielectric material willprovide the needed hermeticity for the additional porous dielectricmaterial.

In a preferred embodiment, the additional porous dielectric material isinterposed between the metallurgical feature 64 and the sintereddielectric material 62 as shown by 68. In the most preferred embodiment,there is this layer 68 of additional porous dielectric material as wellas layer 70 of additional porous dielectric material covering themetallurgical feature so that the additional porous dielectric materialentirely encapsulates the metallurgical features.

The additional porous dielectric material may comprise ceramic materialand a sintering retardant, as explained previously, in order to form theporosity. Alternatively, the additional porous dielectric material maycomprise hollow glass microspheres.

It is contemplated that the previously described embodiment of FIG. 7may, and often will, be used in conjunction with the previousembodiments of the invention where a polymeric material is disposedwithin the substrate. In particular, the polymeric material may be usedto fill permeable pores in the metallic vias or simply to fill thepermeable voids resulting from the mismatch in thermal expansions of themetallic vias and the sintered dielectric material.

The embodiments of FIG. 7 may be made by the tape casting process asdescribed previously. Thus, the steps comprise mixing a slurrycomprising at least a dielectric material, a binder and a solvent;casting the slurry into a plurality of greensheets; forming a pattern ofvia holes in the greensheets; filling the via holes with a conductivematerial; applying a plurality of conductive patterns on at least one ofthe greensheets, wherein each of the patterns comprises a metallurgicalfeature at least partially encapsulated by selectively disposedadditional dielectric material; stacking the greensheets in alignmentwith each other; laminating the stack; and then firing the stack to atemperature sufficient to sinter the dielectric material and themetallurgical features but not sufficient to densify the additionaldielectric material such that the additional dielectric materialcontains porosity.

The step of applying a plurality of conductive patterns may comprisefirst applying the metallurgical features on the the greensheetsfollowed by applying the additional dielectric over the metallurgicalfeatures. The end result of this method would be to have metallurgicalfeature 64 and additional porous dielectric 70. The metallurgicalfeatures and the additional dielectric material may be applied byconventional screening methods or by other methods such as decaltransfer, lithography, etc.

Alternatively, the step of applying a plurality of conductive patternsmay comprise first applying the additional dielectric material on thegreensheets followed by applying the metallurgical features on theadditional dielectric material followed by applying additionaldielectric material over the metallurgical features. The end result ofthis method would be to have metallurgical features 64 encapsulated byadditional dielectric material 68 and 70.

Additional dielectric material 68 may be simply screened, for example,on the surface of the greensheet as shown in FIG. 7A. Alternatively, theadditional dielectric material 68 and/or the metallurgical feature 64may be disposed in a cavity within the greensheet as shown in FIGS. 7Band 7C. In FIG. 7B, a cavity 72 is formed in the greensheet by a laser,for example, and then additional dielectric material 68 is disposedtherein. Thereafter, the metallurgical feature 64 and additionaldielectric material 70 are disposed over the additional dielectricmaterial 64. In FIG. 7C, cavity 72 is formed in the greensheet foradditional dielectric material 68. Thereafter, a second cavity 74 isformed in the additional dielectric material 68 for metallurgicalfeature 64. Finally, additional dielectric material 70 is disposed overmetallurgical feature 64.

While the subject invention has been taught with reference to specificembodiments, the invention is not intended to be limited to thespecifically described materials, structures and processes. One havingskill in the art will recognize modifications and extensions of theteachings within the spirit and scope of the appended claims wherein:

What is claimed is:
 1. A substrate for interconnecting electroniccomponents comprising:a non-porous dielectric body; metallurgicalfeatures associated with said dielectric, said metallurgical featureshaving permeable pores; and a polymeric material within said permeablepores, wherein said polymeric material is a polymeric material selectedfrom the group consisting of polyimides, bismaleimides, acetylenes,epoxies, and thermally-stable silicon-containing polymers.
 2. Amultilayered ceramic substrate for mounting semiconductor devicescomprising:a plurality of layers of sintered dielectric material; aplurality of patterns of metallurgical features disposed on saiddielectric material layers, said features having permeable pores; and apolymeric material within said permeable pores, wherein said polymericmaterial is a polymeric material selected from the group consisting ofpolyimides, bismaleimides, acetylenes, epoxies, and thermally-stablesilicon-containing polymers.
 3. A substrate for mounting semiconductordevices comprising:at least one layer of non-porous dielectric materialhaving a pattern of via openings for receiving metallurgical features;first metallurgical features in said via openings, said firstmetallurgical features having a diameter smaller than the diameter ofsaid via openings; and a polymeric material surrounding said firstmetallurgical features and abutting said dielectric material, whereinsaid polymeric material is a polymeric material selected from the groupconsisting of polyimides, bismaleimides, acetylenes, epoxies, andthermally-stable silicon-containing polymers.
 4. The substrate of claim3, additionally comprising second metallurgical features disposed on thesurfaces of said at least one layer of sintered dielectric material. 5.The substrate of claim 4, wherein said first and said secondmetallurgical features comprise the same metallurgy.
 6. A substrate forelectrically interconnecting components comprising:a sintered dielectricmaterial having permeable pores therein; sintered metallurgy disposed insaid dielectric; and a polymeric material within said permeable pores,wherein said polymeric material is a polymeric material selected fromthe group consisting of polyimides, bismaleimides, acetylenes, epoxies,and thermally-stable silicon-containing polymers.
 7. The substrate ofclaim 6, wherein said sintered dielectric material comprises:a pluralityof layers of sintered ceramic material; and at least one surface layercomprising ceramic material and a sintering retardant, said at least onesurface layer having permeable pores.
 8. In a substrate for mountingelectronic devices comprising dielectric material having metallurgicalfeatures, there being a mismatch in the respective thermal expansions ofthe materials resulting in permeable voids in said substrate, theimprovement comprising:a polymeric material disposed in said permeablevoids, wherein said polymeric material is a polymeric material selectedfrom the group consisting of polyimides, bismaleimides, acetylenes,epoxies, and thermally-stable silicon-containing polymers.
 9. Amultilayered ceramic substrate for mounting semiconductor devicescomprising:a continuous network of metallized dielectric materialcomprising a co-fired glass-ceramic material and sintering retardantthereof, said continuous network having permeable interstices; and aflexible polymeric material disposed in said permeable interstices,wherein said polymeric material is a polymeric material selected fromthe group consisting of polyimides, bismaleimides, acetylenes, epoxies,and thermally-stable silicon-containing polymers.
 10. The substrate ofclaim 9 wherein said sintering retardant is selected from the groupconsisting of silicon nitride, fused silica, mullite, alumina andprecrystallized cordierite.
 11. The substrate of claim 9 wherein saiddielectric material comprises alumina, borosilicate glass, borosilicateglass plus silica, silica or glass-ceramic.
 12. A multilayered ceramicsubstrate comprising:a plurality of layers of sintered dielectricmaterial; a plurality of patterns of sintered metallurgical featuresdisposed on said dielectric material layers; additional porousdielectric material being selectively disposed over at least one layerof said sintered dielectric material and in contact with saidmetallurgical features such that said additional porous dielectricmaterial at least partially encapsulate said metallurgical features. 13.The substrate of claim 12 wherein said sintered dielectric material isnon-porous.
 14. The substrate of claim 12 wherein said additional porousdielectric material entirely encapsulate said metallurgical features.15. The substrate of claim 12 wherein said additional porous material isinterposed between said metallurgical features and said sintereddielectric material.
 16. The substrate of claim 12 wherein saidadditional porous material comprises ceramic material and a sinteringretardant.
 17. The substrate of claim 12 wherein said additional porousmaterial comprises hollow glass microspheres.
 18. The substrate of claim12 further comprising metallic vias in said sintered dielectricmaterial, said metallic vias having permeable pores and a polymericmaterial disposed within said permeable pores, wherein said polymericmaterial is a polymeric material selected from the group consisting ofpolyimides, bismaleimides, acetylenes, epoxies, and thermally-stablesilicon-containing polymers.
 19. The substrate of claim 12 furthercomprising metallic vias in said sintered dielectric material, therebeing a mismatch in the respective thermal expansions of said metallicvias and said sintered dielectric material, resulting in permeable voidsin said substrate, and further comprising a polymeric material disposedin said permeable voids, wherein said polymeric material is a polymericmaterial selected from the group consisting of polyimides,bismaleimides, acetylenes, epoxies, and thermally-stablesilicon-containing polymers.